1. Field of the Invention
The invention pertains generally to etching, and in particular etching practiced in the manufacture of information processing devices.
2. Art Background
During the fabrication of devices such as electronic devices, e.g., integrated circuits, and magnetic bubble devices, many process steps involve the etching of patterns into regions of various composition, e.g., semiconductor material and metals. Typically, device patterns are etched into a region by etching the region through a correspondingly patterned etching mask, e.g., an exposed and developed resist. This etching is accomplished by using, for example, wet chemical etching or plasma etching. An important consideration in all of these etching procedures is control of etch depth. For example, it is often desirable to terminate the etching at a desired depth within a substantially homogeneous material. It is also often desirable, if the region to be etched overlies a layer of a second material, to terminate etching at the interface.
Various techniques have been devised for monitoring etching procedures. One such technique is described in the journal article by Busta et al entitled, "Plasma Etch Monitoring With Laser Interferometry", Solid State Technology, Vol. 22, No. 2, pp. 61-64 (1979). According to this technique a helium-neon laser is directed through a beam expander and beam splitter onto a uniform area of a substrate undergoing etching within a plasma etching chamber, as shown in FIG. 1. The intensity of the light reflected from the substrate is detected and recorded as a function of time. If the substrate layer being etched is a layer of relatively nontransparent material, e.g., a layer of metal, then the recorded intensity-time curve has a constant amplitude (which depends on the reflectivity of the nontransparent material) until the layer of nontransparent material is etched away. At an interface between two different layers of nontransparent material, e.g., at the interface between two layers of metal, the constant-amplitude reflectivity undergoes a single step change indicating the end point in the etching of one of the layers of nontransparent material.
When the material being etched is relatively transparent to the incident light and overlies a reflective surface, then the measured light intensity goes through a series of minima. Because the material is transparent, the incident light is both reflected from the upper surface of the transparent material and is refracted through the material, as shown in FIG. 2. At the reflective surface, the refracted light is also reflected upwardly through the transparent material, exiting the material to interfere with the light reflected from the upper surface of the material. Etching results in a decreasing optical path length through the transparent material and to varying interference conditions. Additionally, at specific thicknesses destructive interference, which corresponds to a relative minimum in the recorded intensity-time curve, occurs, and at other specific thicknesses constructive interference, which corresponds to a relative maximum, occurs. If the incident light impinges the transparent material at normal incidence, then the change in thickness of the material between any two adjacent minima or any two adjacent maxima in the recorded intensity-time curve is equal to .lambda./2n (see Busta et al, supra, at 62), where .lambda. is the wavelength of the laser light and n is the index of refraction of the transparent material. Thus, by measuring the time interval between any two adjacent minima or any two adjacent maxima, the etch rate of the transparent material is determined. Furthermore, by counting the number of cycles or periods in the recorded intensity-time curve, the etch depth is also determined.
The technique described in Busta et al is useful for monitoring depth of etching of a transparent layer of material and in detecting an interface between layers of nontransparent material. However, this technique cannot be used to monitor the etch depth of a nontransparent layer of material.
Another technique for monitoring etching which has only been used for determining the etch rate and etch depth of a transparent layer of substrate material (rather than a nontransparent layer of material) undergoing etching, such as a layer of SiO.sub.2 or a layer of Si.sub.3 N.sub.4, is described in the journal article by Kleinknecht et al entitled "Optical Monitoring of the Etching of SiO.sub.2 and Si.sub.3 N.sub.4 on Si by the Use of Grating Test Patterns," J. Electrochemical Society, Vol. 125, pp. 798-803 (1978). In this technique test patterns in the form of diffraction gratings are defined in an area of the photoresist mask distinct from the region containing the device pattern. Upon shining a laser beam onto one of the test patterns, the light reflected from the test pattern forms a diffraction pattern (a pattern of bright and dark fringes). During the etching procedure, the intensity of the first-order diffracted light (one of the bright fringes) reflected from a test pattern is monitored, and recorded as a function of time. This intensity oscillates with time as the etching proceeds because the phase difference between the light reflected from the photoresist grating bars and the light reflected from the underlying layer of SiO.sub.2 or Si.sub.3 N.sub.4 (the layer of material being etched) varies as the thickness of the SiO.sub.2 or Si.sub.3 N.sub.4 is reduced.
To determine the etch rate of the transparent layer of SiO.sub.2 or Si.sub.3 N.sub.4, Kleinknecht et al uses the Fraunhofer integral, (see, e.g., M. Born and E. Wolf, Principles of Optics, pp. 401-403, Pergamon Press, Inc., Elmsford, N.Y. (1965)), to derive a theoretical formula for the first-order diffracted intensity reflected from a rectangular grating profile. This formula is then used to produce a theoretical curve for the first-order diffracted intensity as a function of the thickness of the transparent layer being etched. The etch rate is determined by comparing this theoretical curve to the corresponding data taken during etching.
The technique described in Kleinknecht et al is disadvantageous because the portion of the substrate containing the test patterns must necessarily be discarded. In addition, this technique provides a measure of the rate at which a test pattern is etched into the substrate, rather than a measure of the rate at which the desired pattern is etched into the substrate. Sometimes, however, the two etch rates are not identical. Finally, the comparison between the theoretical curve and the measured data is complicated and inconvenient, especially since a theoretical curve must necessarily be calculated for each particular test pattern and for each thickness of photoresist.
Accordingly, an important objective of those engaged in the development of the plasma and chemical etching arts, as applied to the fabrication of devices such as electronic devices and magnetic bubble devices, is the development of a technique for directly monitoring etch rates and etch depths of nontransparent materials which is not wasteful of substrate material and which is relatively convenient.